1. Technical Field
The present invention generally relates to ultrasound systems, and more particularly to an ultrasound system and a method of forming elastic images capable of preventing distortion.
2. Background Art
An ultrasound system, which has been widely used in the field of medical diagnosis, transmits ultrasound signals to a target and receives echo signals reflected from the target to form an ultrasound image. A brightness-mode (B-mode) is a representative ultrasound image display mode, which is expressed with reflection coefficients of tissues depending on acoustic impedance differences between the tissues. However, it is difficult to observe a tissue having the reflection coefficient not lager than those of neighboring tissues such as a tumor.
Tissues such as tumor or cancer can be found with the elastography showing the mechanical characteristics of the tissues (i.e., elastography images of tissues), which cannot be diagnosed with the B-mode ultrasound image. Thus, the elastography is very helpful to diagnose resins.
In order to form an elastic image, reference signals are formed with reflective signals from the target prior to applying pressure to the target and receive signals are formed with other reflective signals from the target to which the pressure is applied. The displacement of the target due to the pressure is obtained with the difference between the reference signals and the receive signals, i.e., the delay time between the reference signals and the receive signals.
While forming the ultrasound image by applying the pressure to the target through a probe, the target moves in upward, downward, left or right directions due to the pressure, density distribution of a medium or density of the tissues around the target. Thus, the target is not located on a fixed position in the image frames. That is, in such a case of forming image frames F1, F2 and F3 with applying pressure to target T (shown in FIG. 1), the position of target T is not fixed on one fixed position even though the density of a medium M is uniform. If the variation of the target's position is ignored in forming the elastic image of the target, then distortion is generated.
In order to prevent the generation of distortion, various methods such as an optical flow, a 2-dimensional correlation and a block matching have been adopted to correct the positions of the target in image frames. However, such conventional methods require a significant amount of calculation and take a long process time. Thus, the capability of a processor should be guaranteed above a certain level.